The present description relates to an intake system of an internal combustion engine, and more particularly to control of a variable intake system and a variable valve lift system.
There is known and presented, for example in Japanese Patent Application Publication JP2004-137982A, a variable intake system of an internal combustion engine, which is capable of adjusting an effective length of an intake air passage to be more suitable for a current engine speed. It utilizes a pulse wave that a motion of air in the intake passage just upstream of an intake valve generates. Conventionally, an intake valve begins to open at a beginning of an intake stroke of a cylinder cycle. When a piston in an engine cylinder passes its top dead center and then descends, the intake valve is substantially or effectively open, and air is sucked into the cylinder. Then, the piston descends, and a negative pressure is generated in an intake port throat which is just upstream of the intake valve.
The negative pressure generates a pressure wave. It transmits upstream in the intake passage. Then, the pressure wave reflects at a larger air volume, for example a surge tank, which may be called an upstream end of a transmission path of the pressure wave. Then, it goes back to the intake valve, which may be called a downstream end of the pressure wave transmission path. The intake valve cyclically opens and closes synchronously with a rotation of a crankshaft of the engine. If the pressure wave reaches at the intake valve which is closed, it reflects there and goes upstream again. If a positive pressure portion of pressure wave reaches at the intake valve that is open, more air can be charged into the cylinder, which is called a supercharging effect.
The variable intake system can synchronize the pressure wave reaching at the intake valve with the openings of the intake valve by adjusting the effective length of the pressure wave transmission path in accordance with a frequency of the intake valve opening that corresponds to the engine speed. It can make a positive pressure portion of the pressure wave to reach at the intake valve at timing when it is open. Therefore, the variable intake system can increase the air charge through a range of the engine speed by changing the effective length.
On the other hand, when a desired intake airflow is smaller, in particular, at a lower engine speed, such as during an engine idling, the air charge to the individual cylinder is usually relatively small. The effective lengths of the pressure wave transmission path for the respective cylinders may vary, for example due to constraints of the intake system design. The variation of the effective lengths may cause the air charge to fluctuate between the respective cylinders, because some of the cylinders may have the supercharging effect and some may not. When the desired airflow is smaller, the air charge fluctuation may be relatively greater. It may cause variation of air-fuel ratio in the combustion chamber, for example. In turn, it may cause unstable combustion and output torque fluctuation, and increase noise or vibration of the engine.
Further, if the airflows to the respectively cylinders are independently controlled, for example by adjusting intake valve lifts, the individual cylinder charges may fluctuate from each other due to mechanical variations of the components, for example. When the desired airflow is smaller, the fluctuation is great relatively to the airflow. When it combines with the cylinder charge fluctuation caused by the variable intake system, the total cylinder charge fluctuation may increase significantly. Therefore, there is a need to improve the variable intake system in terms of the operation for the smaller airflow.